1. Field of the Invention
The present invention relates to an image recording apparatus such as an electrophotographic color copying machine and a laser printer and more particularly to an optical scanning device for use in the image recording apparatus.
2. Description of the Prior Art
In an electrophotographic color copying machine or the like, an optical scanning device (laser beam scanner device) using a semiconductor laser is often used as a means for forming an electrostatic latent image on a photosensitive image forming body by an image signal corresponding to an original.
When a laser beam scanner device is employed, a compact, inexpensive semiconductor laser can be used as a light source for an image forming apparatus, and a clear color image can be recorded.
FIG. 1 shows an example of a laser scanner device 10 using a semiconductor laser, wherein a post-objective type optical system is used as an optical system.
The post-objective type optical system is an optical system in which a beam is focused by a condenser lens, and is then incident on a light deflector, as will be described later.
In FIG. 1, a semiconductor laser 11 is optically modulated based on a drive signal corresponding to color separation data (e.g., binary data) obtained from a driver 12.
A laser beam emitted from the laser 11 is incident on a light deflector 15 through a collimator lens 13 and a condenser lens 14 as a focusing element.
The laser beam deflected by the light deflector 15 is radiated on the surface of an image forming body (photosensitive drum) 16. In this case, the surface of the image forming body 16 is scanned in a predetermined direction a at a constant speed, so that an exposed image corresponding to the color separation data is formed on the image forming body 16.
After exposure, an electrostatic latent image is developed by toner supplied from a developer (a plurality of developers are provided in the case of a multi-color image), and is then fixed, thereby recording an image on a recording sheet.
Note that reference numeral 17 denotes a photosensor, which receives the laser beam reflected by a mirror 18 so as to obtain an index signal indicating the beginning of scanning of the laser beam. Image data for one line is written based on the index signal.
When the post-objective type optical system is used as an optical system, since a beam is subjected to convergence before deflection, the condenser lens 14 poses only a problem of axial image formation, resulting in a simple structure.
However when the post-objective type optical system is used, a convergent point is normally located on a curved plane, as indicated by a broken line in FIG. 2. Therefore, in practice, the curvature of field must be corrected by any means.
A pre-objective type optical system is normally used in consideration of the above problem.
As shown in FIG. 3, in the optical system of this type, a condenser lens is omitted, and instead, focusing lenses 20 are arranged between the light deflector 15 and the image forming body 16.
As the focusing lenses 20, a sin.sup.-1 .theta. lens constituted by two lenses as in this example is used.
When the pre-objective type optical system is used, the focusing lenses 20 must be arranged to meet the following requirements. That is, an image must be precisely formed on the image forming body 16, a beam must scan the image forming body 16 at a constant speed, the curvature of field and distortion must be reliably corrected, and so on. The focusing lenses must be arranged to function as a wide-angle lens capable of covering a beam deflection angle.
Therefore, in a beam scanner device having the preobjective type optical system, the focusing lenses 20 must be precisely machined, resulting a very expensive optical system.
In the post-objective type optical system, an inexpensive condenser lens can be used, while some countermeasure for a curvature of field must be taken. When the pre-objective type optical system is used, although the curvature of field can be effectively corrected, an expensive focusing lens must be used.
According to the present invention, it is possible to flatten a focusing surface with a single lens which is arranged between the light deflector and the surface to be scanned. There is an example of flattening of the image surface by diverged light incident on the light deflector and by use of Petzval sum of the single lens, but angle of view of as small as 16.degree. is obtained. There is another example of flattening of only curvature of field with a single lens by ignore Coma aberration, as an optical system of a laser beam printer has small aperture ratio. However, surface shape is complicated and machining is difficult.